Geology Reference
In-Depth Information
Fig. 15.19
Close-up view of mudstone-draped foresets within a large-scale sandwave deposit, Moodies Group, Eureka Syncline.
Note the cyclic thickening and thinning of mudstone-draped foreset laminae. Scale is in centimeters
BIF is supported by the local presence of intercalated
volcanic ash beds (Trendall and Blockley
1970
).
Because some geyser activity today is modulated by
earth tides (Rinehart
1972a, b
), the question is raised
by Williams (
2000
) as to whether the Weeli Wolli
cyclicity records earth-tidal rhythms that modulated
the discharge of silica- and (or) iron-bearing fumarolic
waters.
Two possible tidal interpretations of the Weeli
Wolli cyclicity are suggested by Williams (
2000
): (a) the
mineral couplets are semidiurnal increments grouped
in lunar fortnightly cycles. By this interpretation, there
were about 28-30 lunar days per lunar month at about
2,500 Ma; (b) the mineral couplets are lunar fortnightly
increments that are arranged in annual cycles related to
seasonal influences on sedimentation. This would indi-
cate about 28-30 lunar fortnights, or about 14-15 lunar
months, per year at about 2,500 Ma. Williams (
2000
)
favors the latter interpretation because geothermal
areas usually are so sluggish mechanically that the
semidiurnal and diurnal components are filtered out,
whereas the activity of geysers may be influenced
by the fortnightly tidal component (Rinehart
1974
).
Furthermore, an annual origin for the Weeli Wolli
cyclic stripes gives sedimentation rates for the com-
pacted facies that are comparable to presumed rates for
other iron-formations in the Hamersley Group in which
microbanding is regarded as annual (see Trendall
and Blockley
1970
; Trendall
1983
). Such an origin
for the cyclicity also finds support in the presence
of between 15 and 27 laminae (depending on the
observer) in a thick microband, presumed to repre-
sent 1 year of accretion, from the Brockman Iron-
formation (Fig.
15.21
; Ewers and Morris
1981
).
15.4
Microbially Induced Sedimentary
Structures (MISS)
Microbial structures in carbonates and cherts are
formed by mineral precipitation whereas equivalent
structures in siliciclastic lithologies originate by the
physical interaction of benthic microbiota with erosion
and deposition of sediment (Noffke et al.
2003a
).
Microbial mats respond to erosion by biostabilization
or react to deposition of sediment by baffling, trapping
and binding (Noffke and Krumbein
1999
; Noffke
et al.
2003a
). This distinctive biotic-physical interac-
tion creates a variety of characteristic sedimentary
structures that, due to their unique mode of formation,
have been categorized as their own group termed
'microbially induced sedimentary structures - MISS'
(Noffke et al.
2003a
). MISS have been described from
a number of Neoproterozoic tidal flat to shallow shelf
